1. Field of the Invention
The present invention relates to a PWM (Pulse Width Modulation) motor drive circuit. More particularly, the present invention relates to the PWN modulation motor drive circuit having a capacitor parallel-connected between a drive IC member and a PWM converter circuit so as to prevent the motor from entering a high-speed mode while starting.
2. Description of the Related Art
Referring to FIG. 1, a conventional PWM motor includes a PWM motor drive circuit 1 electrically connected with a motor coil 2 so as to carry out alternatively magnetizing the motor coil 2. The alternatively magnetized motor coil 2 can drive a motor rotor (not shown) to turn with respect to a motor stator of the motor. Typically, the PWM motor drive circuit 1 includes a drive IC member 10, a Hall IC member 11 and a PWM converter circuit 12. The drive IC member 10 electrically connects with the Hall IC member 11 so as to permit drive IC member 10 to receive rotor-detecting signals generated from the Hall IC member 11. However, the drive IC member 10 is designed to have a pin VTH which electrically connects with the PWM converter circuit 12. Correspondingly, the PWM converter circuit 12 has a PWM input pin 121 serving to introduce a PWM signal from an exterior system (not shown). The PWM signal is converted into a voltage signal by the PWM converter circuit 12, and then sent it to the pin VTH of the drive IC member 10 for controlling or adjusting a period of alternative magnetization of the motor coil 2. Accordingly, operational statuses of the motor are possessed of multi-speed modes in heat-dissipating operation by means of the PWM signal.
Generally, the motor divides the operational statuses into a high-speed mode (including full speed), a low-speed mode (excluding full or zero speed) and a stopping mode (zero speed). The drive IC member 10 can determine the operational statuses of the motor according to the input PWM signal such that the motor can be adjusted and changed in speeds to fulfill various system needs. For example, when a voltage of the pin VTH of the drive IC member 10 is higher than 3.6 V, the drive IC member 10 controls the motor to operate at the stopping mode as well as zero rpm. Conversely, when the voltage of the pin VTH of the drive IC member 10 is lower than 2.0 V, the drive IC member 10 controls the motor to operate at the high-speed mode as well as 6,000 rpm. If the voltage of the pin VTH of the drive IC member 10 is in the range of 2.0 V to 3.6 V, the drive IC member 10 controls the motor to operate at the low-speed mode as well as greater than zero rpm but lesser than 6,000 rpm.
Referring again to FIG. 1, the PWM motor drive circuit 1 is designed to have a capacitor 3 parallel-connected between the drive IC member 10 and the PWM converter circuit 12. Meanwhile, the capacitor 3 is designed to have a ground connection in place. In operation, the capacitor 3 is adapted to commutate a saw tooth wave input from the PWM converter circuit 12. However, the capacitor 3 of the PWM motor drive circuit 1 is so configured to stabilize the voltage of the pin VTH of the drive IC member 10. When the motor is actuated, the voltage of the pin VTH of the drive IC member 10 can determine and adjust the speed of the motor.
Referring to FIGS. 2A and 2B, the drive IC member 10 can control the motor to operate in the high-speed mode or the low-speed mode. In normal operation, the speed of the motor is operating at 2,000 rpm as well as low-speed mode when the voltage of the pin VTH of the drive IC member 10 is maintained at 3.0 V (i.e. lesser than 3.6 V but greater than 2.0 V). But, in abnormal operation, the speed of the motor is operating at high-speed mode when the voltage of the pin VTH of the drive IC member 10 is dropped to zero volts (i.e. lesser than 2.0 V).
Still referring to FIGS. 2A and 2B, due to a ground connection, the voltage across the capacitor 3 is generally zero volts, as best shown in FIG. 2A, and the capacitor 3 can be charged by a voltage from a power supply when the motor is started. Inevitably, the voltage of the pin VTH of the drive IC member 10 is maintained at substantially zero volts. In this way, the drive IC member 10 can invariably control the motor to operate in the high-speed mode as long as the motor is started; namely, the speed of the motor is rapidly and shortly jumped to 6,000 rpm (i.e. full speed) from zero rpm, as best shown in FIG. 2B.
Referring back to FIGS. 1 and 2B, once started, the motor must inevitably enter the high-speed mode that must rapidly and shortly increase the speed of the motor. However, there is no greater amount of operational heat for dissipation. This results in the motor unnecessarily operating at full speed (i.e. top speed) that generates an increased amount of air noise and vibration. Furthermore, the motor occurs an increased amount of abrasion among motor components that may shorten the longevity of the motor.
Referring again to FIGS. 2A and 2B, the voltage across the capacitor 3 can reach 3 V in the event after charging for a predetermined time. In this way, the voltage of the pin VTH of the drive IC member 10 is greater than 2.0 V but lesser than 3.6 V so that the drive IC member 10 terminates the motor to operate in the high-speed mode. Accordingly, the speed of the motor is dropped to a predetermined speed or a lower speed of 2,000 rpm.
However, ambient heat generated from a heat source is lower than a high temperature when the motor is started. Therefore, it is undesirable to permit the drive IC member 10 to increase the speed of the motor reaching 6,000 rpm in the high-speed mode that is unsuitable for the need of normal usage or an improper usage of the motor due to a waste of power consumption. Hence, there is a need for improving the motor to prevent entering the high-speed mode while starting.
The present invention intends to provide a PWM motor drive circuit having a capacitor parallel-connected between a drive IC member and a PWM converter circuit. The capacitor further electrically connects with a power source so that a voltage of a pin VTH of the drive IC member cannot be started at zero volts while starting the motor. Accordingly, the voltage applied to the pin VTH of the drive IC member can prevent the motor from entering a high-speed mode while starting the motor in such a way as to mitigate and overcome the above problem.